The invention relates to a fuel injection system, in particular a common rail system, with a pre-feed pump that delivers a fuel flow from a fuel tank to a high-pressure pump, wherein a fuel filter is disposed downstream or upstream of the pre-feed pump in terms of the feed direction.
Failures of the pre-feed pump and the high-pressure pump have occurred in the operation of fuel injection systems of this type. In tests carried out within the scope of the current invention, it has turned out that the failures of the high-pressure pump are due to an insufficient supply of fuel.
The principal object of the current invention is to produce a fuel injection system in which the service life of the pre-feed pump and the high-pressure pump is increased.
A further object of the invention is that damage to the drive mechanism of the high-pressure pump should be prevented in the event of an insufficient supply quantity.
In a fuel injection system, in particular a common rail system, with a pre-feed pump that delivers a fuel flow from a fuel tank to a high-pressure pump, in which a fuel filter is disposed downstream or upstream of the pre-feed pump in relation to the feed direction, the inventive object is attained by means of a safety switch that is dependent on the fuel filter pressure difference. The supply quantity of the high-pressure pump is essentially dependent on the delivery pressure to be produced by the pre-feed pump. As the fuel filter becomes contaminated, the delivery pressure increases as a result of the rising through flow resistance. The delivery quantity decreases based on the pre-feed pump characteristic curve. In the known common rail systems, a clogging of the fuel filter is not detected promptly enough so that the above-mentioned failures can occur. The safety switch according to the invention prevents an impermissible increase in the delivery pressure. As a result, both the pre-feed pump and the high-pressure pump are protected. It is also conceivable to provide pressure sensors before and after the fuel filter with which the pressure differential can be detected.
A particular embodiment of the invention is characterized in that a valve device is provided which supplies the fuel flow delivered by the pre-feed pump either back into the fuel tank or to the high-pressure pump depending on the fuel filter pressure difference. Viewed in terms of the flow direction, if the pressure upstream of the fuel filter increases too intensely as a result of contamination, the fuel flow from the pre-feed pump is no longer supplied to the high-pressure pump, but back into the fuel tank. As a result, the motor is switched off and the high-pressure pump is protected.
Another particular embodiment of the invention is characterized in that the valve device has a valve housing in which a spring-preloaded control piston can move back and forth, and depending on the fuel filter pressure difference, this control piston unblocks a shutoff bore in the valve housing, through which the fuel flow delivered by the pre-feed pump travels to the high-pressure pump, or unblocks a diversion bore in the valve housing, through which the fuel flow delivered by the pre-feed pump travels back into the fuel tank. The shutoff bore to the high-pressure pump is already closed to a large extent before the diversion bore to the fuel tank is opened in order to produce a rapid shutoff when the critical fuel filter pressure differential is reached. A critical pressure differential value can be predetermined by means of the initial stress of the spring. As soon as this critical pressure differential value is reached, the diversion bore in the valve housing is unblocked. When there is a lower pressure difference, the injection system functions normally.
Another particular embodiment of the invention is characterized in that an expansion material element is disposed in a bypass that circumvents the valve device. The safety switch according to the invention would also be triggered by means of gelled fuel at low temperatures. In order to prevent this, the expansion material element is disposed parallel to the valve device. At low temperatures, the expansion material element opens the bypass and assures that the fuel flow delivered by the pre-feed pump reaches the high-pressure pump.
Another particular embodiment of the invention is characterized in that the valve device and/or the expansion material element are integrated into a cover of the fuel filter. This produces a particularly compact embodiment which is suitable for a modular construction.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawing. The features mentioned in the claims and in the description can be essential to the invention each individually or in arbitrary combinations.